SQL - UNION, UNION ALL, INTERSECT, EXCEPT

I am learning SQL with GalaXQL and can't figure out the following question:

Build a query which returns starids from planets. The starids should be selected so that for each starid (x) in the list:
- there should exist a planet with a starid that's three times x but
- there should not exist a planet with starid two times x.
Only use starid:s from the planets table.

What would be a query that would accomplish this using UNION, UNION ALL, INTERSECT, EXCEPT?
Please digest this in your answer as I am a beginner.
Thank you in advance!